The present invention relates to a vertical deflection circuit of a picture display unit which is capable of displaying a picture in accordance with a variety of scanning frequencies.
In a picture display unit used in a computer terminal and the like, display position and display size of picture or scanning frequency, etc. on a scope have wide variety in recent years. Therefore, there has been such a problem that production in small quantities for a wide variety has to be performed in accordance with a plurality of input signals, thus deteriorating productivity.
Thereupon, picture display units which are capable of displaying pictures corresponding to a variety of scanning frequencies have been proposed in order to meet the variety of scanning frequencies as described above.
In such a picture display unit, however, there has been a problem that, when an ordinary sawtooth-wave generating circuit is used as a vertical oscillation circuit of a vertical deflection circuit, the amplitude of the sawtooth voltage which is output from the sawtooth-wave generating circuit is varied, when the vertical scanning frequency is varied, in reverse proportion to the variation of the vertical scanning frequency, thus resulting in variation of the display size in the vertical direction of a picture which is displayed on the scope.
Therefore, picture display units that are disclosed, for example, in Japanese Patent Laid-Open No. 63-131716 (JP-A-63-131716), etc. have been proposed as a conventional picture display unit which had solved such problems.
A conventional example disclosed in Japanese Patent Laid-Open No. 63-131716 (JP-A-63-131716) will be described hereafter with reference to FIG. 1.
FIG. 1 is a circuit diagram showing a principal part of a conventional picture display unit, and those parts shown in FIG. 1 show a vertical oscillation circuit portion in a vertical deflection circuit of a picture display unit.
In FIG. 1, a numeral 111 denotes an oscillator, 112 denotes a capacitor for oscillation, 113 denotes a switch, 114 denotes a vertical synchronizing pulse input terminal, 115 denotes a power source voltage input terminal, 116 denotes a sawtooth voltage generator, 117 denotes a switch, 118 denotes a constant current source, 119 denotes a capacitor, 1110 denotes a sawtooth voltage output terminal, 1111 denotes a comparator, 1112 denotes a peak-hold circuit, 1113 denotes a switch, 1114 denotes a holding capacitor, 1115 denotes a comparator, 1116 denotes a capacitor and 1117 denotes a voltage-current converter.
In this conventional example, when a vertical synchronizing pulse VD is input to the input terminal 114, the switch 113 is changed-over alternately between a terminal A and a terminal B in accordance with the level of the vertical synchronizing pulse VD. With this, the oscillator 111 oscillates, thereby to generate a voltage in the capacitor 112. The waveform of this voltage is a sawtooth waveform, and the maximum voltage level reaches to V.sub.H and the lower limit takes an intermediate level between V.sub.L V.sub.L '. In the oscillator 111, the voltage generated in the capacitor 112 is applied with pulse shaping, thus generating a sawtooth wave generation initiating pulse P.sub.TR of the sawtooth voltage generator 116 and a sampling pulse P.sub.PH of the peak-hold circuit 1112.
In the sawtooth voltage generator 116, when the pulse P.sub.TR is input, the switch 117 is put ON and the capacitor 119 is charged rapidly. Then, when the voltage level across the capacitor 119 reaches to V.sub.H ', the switch 117 is put OFF by means of the comparator 1111 and the capacitor 119 is discharged through the constant current source 118. In such a manner, a sawtooth voltage V.sub.SAW is created and output from the output terminal 1110.
Further, the peak value of the sawtooth voltage V.sub.SAW is sampled by means of the switch 1113 of the peak-hold circuit 1112 and is held by the capacitor 1114. The held voltage is always compared with a reference voltage V.sub.ref which defines the amplitude of the sawtooth voltage V.sub.SAW by means of the comparator 1115, and the error voltage between both is output from the comparator 1115. The output voltage is converted into a current by means of the voltage-current conversion circuit 1117, and controls the current value of abovementioned constant current source 118 thereafter. As a result, the amplitude of the sawtooth voltage V.sub.SAW which is output from the output terminal 1110 becomes constant irrespective of the frequency of the input vertical synchronizing signal. In other words, there is no means for changing the amplitude.
That is, in this conventional example, the amplitude of the sawtooth voltage V.sub.SAW which is output from the output terminal 1110 is always constant even in the case where the vertical scanning frequency is varied. Therefore, it is possible to maintain the display size in the vertical direction of the picture displayed on the scope always constant.
In the above-described conventional example, however, there have been such problems as described hereunder.
That is, the display size of a picture displayed on a scope generally has different sizes which are optimum depending on video signals which are input to a picture display unit. In the above conventional example, however, the reference voltage V.sub.ref which defines the amplitude of the sawtooth voltage V.sub.SAW is fixed and cannot be varied. Thus, there has been a problem that even if it is tried to vary the display size so that the display size in the vertical direction becomes optimum, it is impossible to vary it.
Further, the above-described conventional example is a vertical oscillation circuit for a vertical deflection circuit of a picture display unit as described above. Therefore, a vertical drive circuit, a vertical output circuit, a vertical deflecting coil and so forth are connected at the latter stage thereof as a matter of course, but there is a fear that variation of the display size in the vertical direction is also produced by element dispersion, temperature characteristics and power source voltage variations as a primary factor in abovementioned circuits. Therefore, it is required to adjust the reference voltage V.sub.ref which defines the amplitude of of the sawtooth voltage V.sub.SAW in order to provide the variation, but there has been a problem that it is impossible to provide such variation because the reference voltage V.sub.ref is fixed and cannot be varied as described above.
However, even if the reference voltage V.sub.ref can be varied and the display size in the vertical direction can be varied freely, it is required to partly discharge the charges stored in the capacitor 1114 of the peak-hold circuit 1112 which holds the peak value of the sawtooth voltage V.sub.SAW when it is considered to vary the display size in the vertical direction from a large size to small size for instance. In the above-described example, however, the only outflow path for charges discharged from the capacitor 1114 to flow is to ground through the switch 1113 and the constant current source 118, and moreover, the output impedance of the constant current source 118 is high, thus discharge being performed only gradually. Accordingly, when the display size in the vertical direction is varied, it takes time to settle down at the size to be attained and the follow-up performance is poor.